TwiML tells Twilio what to do when you receive an incoming call or SMS. When
someone makes a call or sends an SMS to one of your Twilio numbers, Twilio
looks up the URL associated with that phone number, makes a request, and then
reads TwiML instructions at that URL to determine what to do, such as record a
call, or send an SMS.

You can use the Twilio REST API to query metadata about your account, phone
numbers, calls, text messages, and recordings. You can also initiate outbound
calls and send text messages.

Send a HTTP POST request to http://your-app-id.appspot.com/call/receive,
which returns the following text:
...
Hello from Twilio!

Copy and paste the http://your-app-id.appspot.com/call/receive URL into the
Voice URL box on the Numbers page of your Twilio Account.

Now call your Twilio number! You should hear a voice say "Hello from Twilio!"
in response. When you call, Twilio will fetch your URL, and execute the XML
instructions listed above. Then Twilio will hang up, because there are no
more instructions.